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666 DYE-SENSITIZED SOLAR CELLS
sheet resistance and high transparency. In addition, sheet resistance should be nearly
independent of the temperature up to 500 ◦ C because sintering of the TiO 2 electrode is
carried out at 450 to 500 ◦ C. Indium–tin oxide (ITO) is one of the most famous TCO
materials. In spite of having low resistance at room temperature, ITO resistance increases
significantly at high temperature in air. Usually, fluorine-doped SnO 2 is used as the TCO
substrate for DSSCs (e.g. Nippon Sheet Glass Co., R = 8–10 /square).
15.1.2.2 TiO 2 photoelectrode
Photoelectrodes made of such materials as Si, GaAs, InP, and CdS decompose under irradiation
in solution owing to photocorrosion. In contrast, oxide semiconductor materials,
especially TiO 2 , have good chemical stability under visible irradiation in solution; additionally,
they are nontoxic and inexpensive. The TiO 2 thin-film photoelectrode is prepared
by a very simple process. TiO 2 colloidal solution (or paste) is coated on a TCO substrate
and then sintered at 450 to 500 ◦ C, producing a TiO 2 film about 10 µm in thickness.
Because this film is composed of TiO 2 nanoparticles (10–30 nm), giving it a nanoporous
structure, the actual surface area of TiO 2 compared to its apparent surface area, roughness
factor (rf), is >1000; that is, a 1-cm 2 TiO 2 film (10 µm thickness) has an actual surface
area of 1000 cm 2 . The dye is considered to be adsorbed on the TiO 2 surface in a monolayer.
Thus, if the nanoporous TiO 2 film has a high rf, the amount of dye adsorbed is
drastically increased (on the order of 10 −7 mol cm −2 ), resulting in an increase of LHE that
is near 100% at the peak absorption wavelength of the dye. In comparison, the amount
of adsorbed dyes on the surface of single-crystal and polycrystal materials is quite small,
with only 1% LHE even at the peak wavelength.
Normally, the TiO 2 film contains large TiO 2 particles (250–300 nm), which can
scatter incident photons effectively, to improve the LHE as shown later. The porosity
of the film is also important because the electrolyte, which contains the redox ions,
must be able to penetrate the film effectively to suppress the rate-determining step via
diffusion of redox ions into the film. Appropriate porosity, 50 to 70%, is controlled in
the sintering process by the addition of a polymer such as polyethylene glycol (PEG)
and ethyl cellulose (EC) into the TiO 2 colloidal solution or paste. Figure 15.2 shows a
scanning electron microscope (SEM) photograph of a typical nanocrystalline TiO 2 film. A
detailed description of the procedure for preparing the TiO 2 film is given in Section 15.2.2.
15.1.2.3 Ru complex photosensitizer
The Ru complex photosensitizer, which contributes the primary steps of photon absorption
and the consequent electron injection, is adsorbed onto the TiO 2 surface. The chemical
structure of typical Ru complex photosensitizers developed by Grätzel’s group are
shown in Figure 15.3 (TBA is tetrabutylammonium cation, (C 4 H 9 ) 4 N + ), and Figure 15.4
shows absorption properties of the complexes in solution. The y-axis is represented by
absorbance (A) and1− T(= 1 − 10 −A ),whereT is the transmittance. The cis-bis(4,4 ′ -
dicarboxy-2,2 ′ -bipyridine)dithiocyanato ruthenium(II) (RuL 2 (NCS) 2 complex), which is
referred to as N3 dye (or red dye), can absorb over a wide range of the visible
regions from 400 to 800 nm. The trithiocyanato 4,4 ′ 4 ′′ -tricarboxy-2,2 ′ :6 ′ ,2 ′′ -terpyridine
ruthenium(II) (black dye) (RuL’(NCS) 3 complex), absorbs in the near-IR region up to